1. Field of the Invention.
The field of the invention relates to the field of MOS transistor fabrication where a buried oxide layer is employed.
2. Prior Art.
Recently a process has been described for forming a buried insulative layer in silicon by implanting oxygen ions. See "Electrical Characteristics of Oxygen Implanted Silicon Substrates", IEDM 84 beginning at page 804 by Foster, Butler and Bolbot. This technique has been used to form a continuous insulative layer which is formed below the surface of the substrate. Transistors are then formed above this buried oxide region. While there are several advantages to this structure, it also has several problems. First, the silicon above the buried layer has a very high defect density (e.g., 10.sup.9 /cm.sup.2) as a result of the damage caused by the implanted oxygen ions. The upper surface of the silicon is also roughened by the implantation, this degrades the carrier mobility at the interface of the gate oxide and degrades the gate oxide. Another disadvantage to this structure is that the channel regions of the transistors are electrically isolated from the silicon by the continuous buried insulative layer causing junctions to become forward biased with the result that the channel conductance is no longer controlled by the gate electrode.
In one prior art process an epitaxial-like silicon layer is formed over insulative regions. The resultant structure resembles a buried oxide region, however, with openings through the buried insulative layer at the site of the channel regions. This places the channel regions in electrical contact with the substrate and overcomes some of the disadvantages mentioned above. The openings in the insulative layer are used as seed windows to propagate the monocrystalline structure of the substrate into a polysilicon layer, thereby forming the epitaxial-like layer over the insulative regions. One problem with this structure is that the openings in the insulative layer are not self-aligned with the channel regions of the transistors. That is, the gate electrode mask is optically aligned to the openings on the silicon wafer by photolithographic equipment. The misalignment tolerance for this procedure is currently approximately plus or minus 0.25 microns. For channel lengths below 1 micron, this misalignment can be a problem. This process is described in copending applications entitled "Process for Forming Isolated Silicon Regions in Field-Effect Devices on a Silicon Substrate", Ser. No. 700,607, filed Feb. 11, 1985, and assigned to the assignee of the present invention.
As will be seen, the invented process provides a structure which has some of the advantages of the above described process and structures without some of their disadvantages.
Other references dealing with oxygen implantation into silicon are: "CMOS DEVICES FABRICATED ON BURIED SiO2 LAYERS FORMED BY OXYGEN IMPLANTATION INTO SILICON", Electronics Letters, pages 593-594, Aug. 31, 78, Vol. 14, No. 18, by K. Izumi, M. Doken, and H. Ariyoshi; "FORMATION OF ABRUPT INTERFACES BETWEEN SURFACE SILICON AND BURIED SIO2 LAYERS BY VERY HIGH DOSE OXYGEN-ION IMPLANTATION", Japanese Journal of Applied Physics , pages 1005-1006, Vol. 19 (1980), No. 5, by Takayoshi Hayashi, Hamao Okamoto and Yoshikazu Homma; "CHARACTERISTICS OF MOSFETS FABRICATED IN SILICON-ON-INSULATOR MATERIAL FORMED BY HIGH-DOSE OXYGEN ION IMPLANTATION", Electronics Letters , pages 356-358, May 14, 1981, Vol. 17, No. 10, by H. W. Lam, R. F. Pinnizzotto, H. T. Yuan, and D. W. Bellavance; "MULTIPLE SOI STRUCTURE FABRICATED BY HIGH DOSE OXYGEN IMPLANTATION AND EPITAXIAL GROWTH", Japanese Journal of Applied Physics , Vol. 20, No. 12, December 1981, pages 909-912, by Yukio Irita, et al; "SIMOX TECHNOLOGY FOR CMOS LSIs", 1982 Symposium on VLSI Technology, Kanagawa, Japan by Katsutoshi Izumi, et al; "FORMATION OF OXIDE LAYERS BY HIGH DOSE IMPLANTATION INTO SILICON", Mat. Res. Soc. Symp. Proc. Vol. 27 (1984) by S. S. Gill and I. H. Wilson; "CHARACTERISATION OF DEVICE GRADE SOI STRUCTURES FORMED BY IMPLANTATION OF HIGH DOSES OF OXYGEN", Mat. Soc. Symp. Proc. Vol. 27 (1984) by P. L. F. Hemment et al.